The present invention relates to a symbol timing recovery apparatus, and more particularly to an apparatus for recovering a symbol timing of a signal received at a receiver for use in a high-definition television or a high-speed data modem.
A receiver for use in a general full-digital high-definition television receiver or a high-speed data modem performs recovery of a symbol rate or a symbol timing frequency, and tracking of a symbol timing during a signal processing. The signal received in the receiver of the high-definition television has considerable distortion due to non-ideal characteristics of a channel, a multi-channel and due to noise during transmission. On the other hand, in the high-speed data transmission system, a phenomenon of symbol timing drift can occur between a transmitter and a receiver. Here, the symbol timing drift means that the number of the symbols transmitted from the transmitter and the number of the symbols recovered in the receiver are different from each other. When the transmitter, which transmits the symbols in a certain symbol transmission period of time T.sub.1, transmits the symbols in a slightly changed period of time T.sub.1 +.DELTA.t due to instability in the circuitry of the transmitter, and when the receiver ignores the change of the symbol transmission period of time and samples the signals in a constant period of time T.sub.1, or to the contrary, when the circuitry of the receiver which samples the signals is unstable, the phenomenon of the symbol timing drift occurs.
Such a distorted signal can be recovered to a degree through digital signal processes such as adaptive carrier recovery, adaptive equalization, etc., in high-definition television. However, such recovery of the received signal is possible only when the received symbol is sampled in the same interval and substantially same phase as those of the symbol transmitted from the transmitter. Therefore, in recovering the received signal in the receiver, it is crucial to find out an exact sampling point of time from the received signal.
Particularly, when the high-speed data transmission of about 6M symbols per second is accomplished and a separate carrier corresponding to the sampling frequency is not transmitted for the symbol timing recovery, as in the high-definition television, it is not only considerably difficult to find out an exact symbol timing using only the received signal, but also the circuitry for realizing the above exact symbol timing is complicated. Thus, it is required that an apparatus for recovering the symbol timing of the high-speed data modem using a simple structure is to be developed.
A symbol timing recovery circuit is disclosed in a Japanese laid-open publication no. J02070140A published on Mar. 9, 1990. When a bit rate of the demodulation data is lowered, this prior art reference discloses a technology for supplying the sampling clock corresponding to the lowered bit rate to an analog-to-digital converter in a digital demodulator. Referring to the above prior art reference, a phase difference between the demodulation data and the sampling clock is detected, and a high frequency clock supplied from a high frequency generator is divided according to the phase difference. The sampling clock is generated based on a high frequency clock which matches a bit rate of the demodulation data among the divided high frequency clocks.
Another prior art reference for recovering the symbol timing is disclosed in U.S. Pat. No. 5,208,839 which was issued on May 4, 1993. The above prior art reference discloses a symbol synchronizer which introduces a symbol clock reference using a clock which is produced out of the baseband signal which is sampled in various digital signal patterns which include non-return-to-zero (NRZ) and Manchester patterns.
U.S. Pat. No. 5,200,981 issued on Apr. 6, 1993 discloses a quadrature amplitude modulation (QAM) modem receiver. This modem receiver uses a sign which is obtained by twice differentiating the baseband signal and performing a locality decision with respect to whether the baseband signal is concave or convex. A global decision which is obtained by summing the locality decisions in a proper period of time is used for correction of the lead or lag of the timing.